I stuck one end of the tubing in the tranny fluid bottle, and sucked on the other end of the tubing until I had a foot or two of fluid in the line.
(One has to be fairly secure in their hetero sexuality to admit that

)
I believe it's crucial to create an equal restriction on both sides of the line, or two things happen:
1 - All the fluid gets sucked out by the left carb, especially if the AI is freshly removed
or
2 - There is so much bounce that it's tough to lock into a reading that won't climb on one side.
I found some plastic tubing couplers in the fastener section of my local hardware store that are multi fit - they start out small, and one snaps off the smaller bores as needed. I do believe the small sections could make a sound means to creating equal line restriction within the home made balancer. I tried wall board lags with limited success.