OK, so I may have it wrong; the following text is copied from the BSA workshop manual.
The cams are of special design. Because of this it is essential that when checking or adjusting the clearance of any valve it should be closed, with its tappet on the base circle, or neutral portion of the cam. To obtain this position for the drive-side inlet valve turn the engine until the gear-side inlet valve is fully open. Similarly, to set the gear-side inlet valve in the correct position, turn the engine until the drive-side inlet valve is fully open. Follow the same procedure exactly for the two exhaust valves.