The SoapmakingFriend calculator is awesome for that! You can put in your MB lye concentration, and then your final desired concentration, and it gives you the correct amount of both the MB lye solution and the additional liquid. If it weren't for that convenience, I'd probably make my solution each time, too.
Since I use SoapCalc and didn't want to have to do a lot of math on the weekends (not that it's that hard...I'm just lazy), I just went with 'ready-to-use' Lye Solution. The advantage is that is literally...ready to us. The disadvantage was hubby wanted more 'bubbles' so I had to add a little more water so I could dissolve sugar.
