Question on pacing strategies:

I understand and have experienced the benefits of negative splitting, starting a little easy, and building the effort as I go, but it's always been by perceived effort and sometimes HR. I've finally gotten off my lazy ass and set up my SRM on the TT bike. Do you guys have a certain formula for your pacing? i.e. 90% FTP for the 1st third, FTP for the 2nd, and 110% or what ever is left in the tank for the 3rd? I'm curious how much you find you need to hold back, without holding back too much. Obviously, this will vary and I need to find what's optimal for myself, but I could use some starting grounds.