Not sure if this will work for you, but from my hobby-level use it seems very accurate and has a lot of very useful features:
Radio Mobile Online. It takes some messing with and tweaking to get accurate maps, but after running a handful it's pretty easy to get the hang of. I find it to be quite accurate. It is designed for ham use and GMRS, but say you want to know the coverage of a UHF repeater, say, at 451 MHz... while it won't let you select 451 MHz exactly, it will let you input 462.5625 (GMRS freq) which will not give a noticeable difference. For 800 MHz, you can input 903 MHz which will be pretty close to 800 MHz coverage.
At first I was getting maps showing a good deal more coverage than I new existed from real world experience and monitoring. It turns out that "required reliability" field makes a huge impact on the coverage map. Running it at 98 or even 99%, which might sound too high, actually gives a pretty accurate map of HT coverage. I ran it at 80% and it was much too generous.
Just my .02!
(I'll also attach a photo of a coverage map I ran for a nearby UHF NXDN repeater. I estimated a few values since I don't know all the antenna details and such, but it came out pretty accurate! I ran it at about 99% for HT-level coverage.)