Soon I will be doing my first BIAB after tweaking the All Amarillo recipe with BIABacus to fit my kettle.  I have a kettle with a sight gauge and "calibrated" it after first getting the kettle two years ago and found the sight gauge measurements to be off from the measured qts/gal that I added to the kettle, so I marked the sight gauge accordingly.  How are most BIAB brewers dealing with measuring your water levels?  I've seen recent posts from Mad Scientist with his Spike kettle on a scale and a formula he has derived in order to mark his sight gauge.  Are a number of you weighing your water? 

MS - I have tried to locate addl info regarding how to use Sections T & V, but haven't been successful.  Can you point me in that direction? 
It looks like your kettle is new.  I've got one of Ben's original generation kettles & have communicated with him regarding electric if BIAB works out for me.  What size is your kettle & whose controller did I see on the wall in one of the pictures?

I have an 18 inch stainless ruler calibrated in English and metric that I got at the hardware store. I dip that into my kettle to measure the depth. With my calibrated kettle I can find the volume at the various control points in the spreadsheet. Low tech but cheap and effective.

Lumpy & Dennis U - thanks for the replies.  Opened up my original recipe which called for 8.06 gal of TWN and "backed into" how to get  by using the headspace calculator.

I understand from the responses I recd re. kettle headspace how to utilize BIABacus to gauge my TWN when filling the kettle.  I have been fermenting my extract brews in a conical and although there are internal etchings in it, they are at the gallon & half gallon levels.  I'd like to hear how others are measuring VIF.  One thought was measuring the kettle headspace after transferring to the conical, but then the trub is still in the bottom of the kettle and I would think that would affect the accuracy of the measurement.
Thanks