the folks at dsd plus told me to use this command line for p25 phase 1. Helped a lot.
DSDPlus.exe -f1 -ds62 -dd97 -dv26
also type capital "S" to disable shaping filter.
You get the correct values by using dsdtune.exe
The -f1 locks it to only do P25 and -fr to only decode DMR.
No -f parameter makes it autodetect, which should be fine if the signal isn't buried in the noise and needs all the help it can get.