I go half and half for couple weeks veg/flower nutes erring on the flower at end of transition, maybe 3/4 dose of flowering for first, this assuming that you have a healthy plant of course
I keep the same strength, but gradually switch by using both for a couple of weeks. Like if I am feeding 10ml per gallon of veg nutes, I might run 7.5 ml veg + 2.5 ml flower the first week of 12/12. The next week cut it down to 5ml veg + 5ml flower, then 2.5ml veg and 7.5ml flower, then to 10ml flower. Keeping slightly higher level of nitrogen the first one to three weeks of 12/12 will help development and prevent half dead plants that seem so common in flower.