Good question! In a lot of ways it's a matter of personal preference in combination with the particular nutrient system you're running. Every one is going to have slightly different levels of N-P-K ratio and as a result it's often necessary to tailor the feeding while in transition to both the plant as well as the nutrient brand.
As a general rule however, I've found better results by staggering the switch between base veg. and flower nutes during weeks 1&2 of the stretch. So after the flip to 12/12 I'll usually keep the plant on a veg. nute regimen during the 1st week of transition. Essentially trying to load 'er up on Nitrogen so that she has what she needs to power out more leaves and thicker branches. Then during the 2nd week I'll drop the base veg. nutes back by half and add in a base flower nute by roughly the same amount. By week 3 of the stretch and/or once she starts showing pistils and true budlets, I'll be switching her over to a full regimen of base flower nutes. Come week 4 of flower, that's when I start adding in the flowering supplements to rev out the P&K as she starts to bulk up with new buds.
That's just what I've found to work most effectively. Hope it helps and good luck with the transition!👍