/diy/ - Do It Yourself

>>wrap string around shelf
>>let plumbob hang
From where? Over the front edge, the rear edge? And where on that edge?...in the middle?
or do you mean over one end?
>>measure from top string and plumbob
There's only one string so "top string" makes no sense.
Also "measure from" those points TO WHAT?
>>when the numbers are the same top and bottom its level
"Top and bottom" OF WHAT?
Do you even know what the words "shelf" or "level" even mean?

There *is* a way it might be done to a reasonable degree of accuracy, but that jabber isn't even close to describing it, and its a tedious process prone to errors.
The ancients weren't stupid and understood that water naturally seeks level and exploited that phenomenon to determine level for large projects like buildings. They had smaller levels that used a plumb bob but not like you describe it.