Anything to note before I file my saddle slot?

Branshen

My acoustic u-bass puts out uneven signal, the E string is softer than the others, and the G string is louder than the others. I believe I have identified the problem which is an uneven saddle slot which means that the piezo pickup is getting uneven pressure from the saddle and strings. 

i'm thinking of buying a saddle slot leveller and fixing the problem myself. I intend to gently square off a portion of the bottom of the saddle slot, which is raised relative to the rest of the slot with this tool:
http://completeguitarsupplies.com/product/hosco-professional-luthier-tools-saddle-slot-files-levelers/?wmc-currency=GBP

If this is a good or a crap tool, let me know. And also, should I get the 3mm or 2mm leveller? I'm inclined to get the 3mm one as the slot for a ubass is much wider than a slot for an acoustic guitar. 

If anyone knows, I'd love any other tips and pointers on how to go about doing this. I suspect @ICBM has dealt with this issue numerous times..

ICBM

You might be surprised to know, no... it’s a fairly rare problem. If it’s definitely not the saddle, I’ve done it totally by hand using a slot chisel and a straightedge.

Branshen

@ICBM definitely not the saddle. It's plastic, so perfectly straight and the piezo strip is glued to it!

I wouldn't dream of taking a chisel to my instruments (hence the leveller file). I'm nowhere near handy enough to do that.

Thanks for the feedback!