In the Speer Manual there is a reduced using 5744 for the 243 win. there is a load for a 80 gr. bullet and 90 gr. I found some inexpensive 85gr. sierra bullets can I simply look at both sets of data in the manual and start in the middle of the two?

Reasonable? Don't know. Would I do what you are proposing. Probably

Using the same reduced load for 80, 85 and 90 QL only calculates a 4000psi difference between the 80 and 90 with the 85 falling right in the middle.

So yes I would split the difference.