Others have already commented on the "true ISO' factor...

However, I've always shot it at 400 in my folders. I have developed it in Rodinal 1+100 and got very nice results.

I've also shot Rollei Retro 400 in 35mm (same as APX400) and rated that at 400 too, but this time developed in PC-TEA 1+50. Once again, nice negatives.