I don't know the first thing about 300 Blackout, but I do know pressure curves aren't linear.
Assuming the bullet is fairly close in shape / bearing surface and all other factors are similar (oal, primer, not using IMI crazy thick brass, et cetera), you can always use data for heavier bullets and potentially take it a little higher. More often than not, your most accurate load won't be at max anyway.
Go slow, work up carefully checking for pressure signs (not just primers flattening). You'll be fine.
Once you get outside things that have been mapped out for 30 years, it really gets interesting. I've loaded some oddball lead profiles and some rifle bullets way longer than they should be for the weight, and it does make you want to cup and turn before you touch off the first one. Again, just always approach it with some common sense and error on the side of caution, and you'll be fine.
I'm not interested in achieving top velocity here. Just looking for some affordable plinking rounds. They do need to reliably cycle the action though.
With powder and bullets a little hard to come by right now I'd rather not waste a bunch of it experimenting or working up a reliable load.
Ha. I forgot I have some older reloading manuals stashed away. Found quite a bit of data for .300 Whisper. Think I'll use that info and go from there.
Loaded up some 147 and 150gr stuff yesterday. Might try to get some 168gr loads put together this evening.