Can I convert stereo to mono for a phono input

A decent cartridge should have inherent channel balance within less than 2db,

In my system a 0.25dB imbalance moves the center of the sound stage by about a foot. Read manufacturers specs on their cart. channel balance. Like Lew wrote, most only guarantee ~2dB or better.

 If your system is all tube with no feedback like mine, there's another "can of worms". As tubes age their gain will change and without feedback to "steady" things balance can drift around a bit. Can be annoying.

One of the easiest ways to measure crosstalk with a test record like the Analog Productions record is with a calibrated 2-channel 'scope. A DMM will also be responding to noise and the reading will be bouncing around making an accurate reading more difficult. With a 'scope it's easy to see and compare the heights of the crosstalk peaks.

John_Tracy, that is a small difference in output for a significant difference in sweet spot. Mine is around 2 feet to the left, in my estimation. I'm curious whether most balance/volume controls provide that high resolution of an adjustment. I would guess 0.5 db is the lowest.

My experience has been that 0.25dB steps for a balance control is about as "wide" as one would want to go. My DIY line stage also doesn't have a balance control. So I provided my DIY power amps with input attenuators that can reduce their gain by ~2dB in 0.25dB steps.