I look to see where any local repeaters might be on, and I check the frequency I want to use with my handheld radio first. In CA, there are basically no repeaters below 440 MHz, so anything below that is pretty much OK.., with only 100 mW of power you're not likely to even be noticed.
Also, be VERY careful about avoiding the AMSAT frequencies. Used to be, folks would put MMDVM hotstpots on 436MHz. That's very commonly used by the sat folks and was causing a LOT of interference. Definitely look to the local repeater coordinator's band plan.
First, consult the applicable band plans. Avoid any small signal, AMSAT, ATV, repeater input/output, designated areas.
Check local usage on FM Simplex. You will likely find little use and plenty of space.
If there is a good sized club in your area, see if they offer any guidance. Here is a page on our club web site, for example: https://rocstock.org/amateur-radio-information/
If you fly with others who use 70cm APRS, find out what they are using. Remember that with time slotting you can have multiple trackers on the same frequency and cooperate to track each other's rockets.
Be sure to research APRS techniques on ham radio sites, also. There is a lot of good information out there.