just threw sum 85% phosphoric acid on the hotplate to concentrate it,   how dry can i safely go?

Just wondering man. It doesn't work for producing HI, or does it?

I mean, I was under the impression it didn't, please correct me if I'm wrong or enlighten me.

Phosphoric + KI --heat--> HI

To raise the concentration above 86% it will require ALOT of heat as the bp goes up exponentially; hence your hotplate will be the limiting factor.

The 86% H3PO4 works perfectly for the aforementioned purpose anyway.  If you want the solid anhydrous H3PO4, buy it.  Dont try and convert from 86% solution.

once it cools down unless stored correctly it will grab for moisture out of the air so it will take away everything you just heated for anyway...

Fairly conc is required but its not to bad, just the only prob is the dissociation of the last hydrogen from phosphoric takes a bit of energy, you may need to heat the mixture to continue the reaction after two substitutions.
1mol phosphoric (make sure your accounting for the dilution) with 3mol KI.

H3PO4 + KI --> KH2PO4 + HI
KH2PO4 + KI --> K2HPO4 + HI

Thats about as far as it may go, so heat a little once you add all of the KI to keep it moving to --

KH2PO4 + KI --> K3PO4 + HI

Just incase it stops bubbling once you add all the KI and pH still shows slight acidic.
Once all three subs are completed solution should be neutral.

-AC