The engine must be idling below 700 RPM to check the initial timing advance. That is because the centrifugal (mechanical) timing advance starts advancing the timing at 700 RPM and is fully advanced by 2300 RPM.
Definition of initial timing: The initial timing is where the crank timing mark is indicated by a timing light with the engine running below 700 RPM, without any centrifugal (mechanical) advance and the vacuum advance disconnected. You cannot check the initial timing on high performance engines because those engines idle too high which is due to the long duration camshafts. The factory idle setting is 650 - 700 RPM on regular production engines and the mechanical timing mechanism on those engines is designed not to function at that RPM. If you have a aftermarket distributor, check with the manufacturer of the distributor on how to check the initial timing advance.
If the engine will not idle below 700 RPM, forget about setting the initial timing advance. Some factory high performance engines had distributors that delayed the start of the centrifugal timing advance until 1100 - 1200 RPM because they have high performance camshafts and the engines will not idle below 900 RPM.