I think you're better off looking at the hot cSt of what you're starting with. Example 5w20 may have a cSt of 8.3, Ur 5w30 might be Cst 10.5, so equal parts give a cSt half way in between
(8.5 + 10.5)/2 = 9.5 then if U check the definition of oil by viscosity, U will see that 9.5 puts it into the VERY thin 30 weight range. 30 weight goes from about 9.3 to 11.4 100 degrees cSt, so you can call what you made 5w25, but more important is what is the hot and cold viscosity, and where Ur pour point ends up, U brewed an oil of cSt 9.3, and then use your judgement from there. For myself, I use this particular mix even for engines that call for 5w30, and they run just fine, and it should work fine also in engines that call for 5w20.