I have never loaded 38 S&W but I have used 148 DeWCs extensively in 38 spl. I like to load them as long as my revolvers chambers will accept. Less bullet jumps and better accuracy. Also lower pressure. In one case I seat them to their halfway point in a 38 Spl case over 2.7gr of Bullseye. They will not chamber in my M10 but when fired from my M581 they are capable of fantastic accuracy. The best 25 yard group I ever fired from a handgun was with that combination.
So to get back to your question; if you already have the powder charge, I would try to figure out how deeply the bullets need to be seated to chamber and that is where I would seat them. My old Lyman manual, circa 1970, gives a 150gr cast over 2.0gr of Bullseye for 598 FPS. Your bullets should be lighter than issue Brit .380 rev ammo and you are using an undersized bullet so pressure shouldn't be too mush of an issue so long as you stick to the book.
