Ideally, yes, charging by weight is the proper way to do it, but I would say just start charging the system and go based on what the temp feels like. When it reaches a level of coolness that you are satisfied with take it for a drive and test it out. Then check your pressures and see what you have. Way I figure, you will hit the proper or ideal charge anyways if the system is operating properly because over charge, it will feel so-so, same as with an under charge. When I charged mine to get the temps that I'm at, that is the method I used. When I went back to check the pressures I was at roughly ideal pressures on low side and high side. This is the method I used yesterday when I helped my friend do his civic....we only put in maybe a full can plus or minus. His system wasn't holding vacuum so since the 152 is so cheap and readily available we charged it anyways until we got the hissing noise of a leak....he had about 4 hardened seals that we replaced and finally no more hissing after 4 separate partial charges.

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1990 190E 3.0L