The cam grinder assumes the rockers will transmit the correct ratio to the valve. So if the cam is sold as having 0.450" valve lift, with 1.5 rockers, the lobe lift will be 0.300" if the cam has been accurately ground.
Is the cam new or used? If used, & lobe peaks are worn, it will be hard to work out original lobe lift.
The cam is used. I bought it in an engine and was trying to decide to use it in my 383 build. It sounds like it's a lot bigger than I want to put in the car. I may have it reground or sell it with the lifters if I can figure what exact size it is.