Trying To Get A Specific Doseage

 
If you can follow the math, I can help you. I also have a formula that can make it fairly simple if you can use a spreadsheet. You can plug in the known parts and it calculates the other parts of the values. Not really hard. What's the ratio of your medical hash oil? If it's like RSO the usual scale uses 1g = 1ml.
 
The reason we convert to ml is because medical hash oil is often placed in syringes instead of weighed but if you have the means to weigh it, that would work, too. Let's use an example for a batch of 20 cookies at 7mg which is 0.007g on a scale. You multiply that 0.007g by 20, the number of cookies, and you'll get a value of 0.14g which is the equivalent of 140mg. All you need to do is measure .14g on a digital scale or .14ml into your butter for the recipe. The same applies for coconut oil or any other carrier as long as you know the volume and weight.
 
I hope this helps. 

 

 
Your scale is mg/g of cookie. Your cookie weighs more than a gram. That may influence your math. In my experience with labs you can't rely too heavily on the results. If you take part of the same sample to another lab, you will get a different reading. I've been doing a lot of testing lately and am totally frustrated with the inconsistency and it's multiple labs. I use HPLC to get the acids and cannabinoids but if I were to use GC, then the readings are higher because the GC just gives the total cannabinoids after the sample was vaporized so the numbers are higher because they're not broken down into the separate components. The dispensaries and clubs around here use the GC numbers because of that and also that's all that's needed for toking. 
 
You should also take the total cannabinoids into consideration, too. They may not add up to a lot, depending on the strain, but they are also a part of the equation since they can influence the effects so much. Unless it's from a large operation like Skunk Pharms or Dixie, the dosage of the edibles are determined just like you did it. That's one of the reasons that products of the same supposed strength have such wide ranging results. If the testing method for the oil was different than used to test the cookie, that could skew the math. 
 
Has your mom tried the cookies? Tell her that you've had the oil tested which allows you to make her specific dose and need her to try one so you can tweak the formula. Assure her that you were conservative with the infusion so it won't be heavier than she'd be comfortable with, it may be weak but you need her input. It may very well be perfect for her. 
 
I have found that my dosing is consistent as long as I know the potency of my source extraction. If you have that, then the math trumps the testing, which is still in it's infancy for edibles. As long as you've mixed the oil into the butter well using a little heat, then there is no question that your cookies are at your goal. There's no place for it to go. It didn't just vanish into thin air. The only disclaimer would be the varied size of a cookie but the gram weight of the cookie dough would contain your desired strength. 
 
If you want to get more exact with your dosage, determine the total weight of your dough, in grams, that contains your desired amount of infusion then divide that by the number of cookies you want, 10 in this case. That will tell you how many grams you need for a consistent dosing for the whole batch. 
 
After she's tried it and it's not just right, then since you know how much oil you used for these, you can either raise or lower a little until you get it just right for her. Or maybe a cookie and a half. When your trying to get a medical dose consistent, it will take a little adjustment to get it just right since everyone is different. Each batch of oil will be different and you'll have to go through the same adjustment period but after a while you'll be able to judge it better and better. 
 
You're on the right track. Believe in your math. Test the oil and the math is foolproof from there. The only inconsistencies can come in the physical blending and sizing of the cookies themselves. 

 

 
The world of lab testing at it's current level is very confusing. Each lab has it's own criteria for samples with no real standardization from one to another yet. Each type of edible or extraction uses a different method. You wouldn't use the same technique, whether by solvent or heat treatment, for a tincture as you would for a solid like a cookie or brownie that doesn't dissolve well. I've seen labs that test for the mg/g of weight and then extrapolate that out for the full weight of the cookie and use that for the total dose. That's fine as long as you know the scale that's being used. That's why I trust using the percentage of the known and reliably tested RSO than the uncertainty of the edibles test. 
 
The small details now don't really matter because now you know what it takes to get the right dose for your mom and that's the most important part. Whatever works.