For one coaching viewpoint on using fractional utilization to adjust training for raising threshold, read the Final Considerations at the end of this article: How to improve your lactate threshold as a cyclist — High North Performance And some of this was mentioned by Coach Chad in an older podcast and my notes are here: Time at VO2max discussion - #11 by bbarrera
Oh, and I still do tempo work at 85-90% but the goal is not raising FTP.