Along the same thinking with the spring off and a new seal to hold the valve in place goto full lift and then push the valve down that the new seal is holding up and use a fine tip marker or wrap a section of masking tape the measure off that.
I always clay them but if I want to get a general idea early on I do the above.
Many ways to get a ball park figure.
Its not the lift, it's the duration. Quick and dirty bring it to TDC overlap. wrap a rag around the rocker arm. Grab the rocker arm with big channel locks and open the valve. How far did it move before it hit? That will get you ballpark.
Doug
