Measure up the wire diameter and number of coils and that will give an idea of what is already in it. If you could easily pull one to measure free length and then compare with measured installed height and a quick drive over the weigh bridge to get an actual weight would allow you to calc it properly but any info is better than nothing for a start. 
Helper bags don't require engineering AFAIK. 
I've got a few sets of springs here that you can play with if you want to come for a drive. 
Steve
				
			 
			
		 
			
				
			
			
				1985 County - Isuzu 4bd1 with HX30W turbo, LT95, 255/85-16 KM2's
1988 120 with rust and potential
1999 300tdi 130 single cab - "stock as bro"
2003 D2a Td5 - the boss's daily drive
			
			
		 
	
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