Het Pet', I think you might be off on the math. Here's how to do it:
Your 1 RM, at a weight of 61 kg, is with a counterweight of 7 kg. So that means the MOST one of your arms can pull is 61-7=54 kg. Now, for the first week, we want the arms pulling 80% of that. So 0.8*54 = 43 kg. So in order to get your arm to pull 43 kg, at a bodyweight of 61, you need a counterweight of 61-43 = 18kg. Do you see how that works?
Second week would be 85%, so thats 0.85*54 = 46. So the counterweight you want is 61-46 = 15 kg.
Third week, the counterweight would be 12 kg
When you deload, I would pick a day in that week to test your new 1 RM. If you can improve by 1 kg every month, you're doing VERY well (that means you can do a 1 RM with 6 kg instead of needing 7 kgs).
I'd be lying if I said I always re-tested. Sometimes, I just took off some weight and went for the new cycle. Once you work on this for a couple of months, you'll have a pretty good idea of how much weight you can reasonably take off.
One last thing. Remember to pull as much as possible on your counterweight WITHOUT the counterweight moving. You WANT to get as much assistance as you've placed on the counterweight, so that means you need to be pulling on it. Do NOT emphasize the working arm by pulling the least you can onto the counterweight. That will make the exercise much harder, will make the sets/reps harder to get, and cause excessive fatigue. You want the assistance.