Hi Bratrules,
I'm fairly new to this game but I have made my own press merely by copying a Dutch Press pictured on the 'net.
Calibration is about as easy as it gets. It will work either for the Dutch Press or the other which you have pictured.
What you need is a set of bathroom scales and a set of kitchen scales.
If you start with the kitchen scales with no weights added to the arm you can tell what pressure is applied just by resting the vertical plunger on the scales. In my case, with no weights, just the vertical plunger and horizontal lever arm in place I had nearly 5 pounds.
Next I replaced the kitchen scales with the bathroom scales and started attaching weights to the end of the pressure arm furthest from the vertical plunger until I achieved a pressure on the scales of 50 pounds (the press will undoubtedly take a lot more weight than this).
I marked this position on the pressure arm and then weighed the actual weights that I had used. In this case it was 7 pounds 13 ounces.
I then moved the weight back along the arm towards the press whilst leaving the bathroom scales under the plunger arm. About a half way along I achieved a reading of 40 pounds, I marked the lever arm at this point and nearer to the vertical plunger the scales read 30 pounds. I marked this position as well.
I now had three marks on the lever arm which related to 50, 40 and 30 pounds pressure assuming that 7 pounds and 13 ounces were applied at these points.
I then placed a mark half way between the 50 pound mark and the 40 pound mark and again added weight until the reading on the scale was 20 pounds. That turned out to be 2 pounds 9 ounces. Again I moved this weight back towards the pivot point and when it reached about half way between the 40 and 30 pound marks I had previously made I got a reading of 10 pounds.
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I now had a means of accurately applying 5, 10, 20, 30, 40 or 50 pounds pressure by just using two weights, 7 pounds 13 ounces or 2 pounds 9 ounces.
I don't even need to keep these weights because all I do is fill plastic bottles with tap water until I have whichever of these two weights I need (as measured on the kitchen scales) and hang them from the arm in a plastic bag. It may not look particularly elegant but I know that I am applying exactly the correct weight.
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Hope this helps.
Guiseppe