Rega arms are not designed for Baerwald alignment. The standard Rega mounting distance of 222mm puts the arm too far from the TT spindle for some cartridges to reach both Baerwald null points. You'd have to move the cartridge farther forward than the headshell slots allow.
If you have a moveable armboard the fix is simple however. Just mount the arm at 219.5-220mm. At that distance any cartridge should reach the Baerwald points without difficulty. I did this successfully with an OL (Rega mount) arm on a Teres (pivoting arm board) and so have many others.
Alternatively, you could use a Rega protractor and mount the arm at the standard dimension. Baerwald alignment tends to produce lower tracing distortion than Rega alignment, however, so that's probably the preferable method.