I might have missed this as am not understanding what projectile, you are going to shoot. Sound like you are going for a PRB. ...
If so and although not "perfect" your 1:48, should still serve you well.
So for those relatively short range shots, is twist rate all that important?
Absotively and twist is to conical as spin is to football. What happens when you put too much spin on a hardwall? That's right, instability, curve or slider. ...
Be Safe !!!