The trick here is, as with all other films, to develop less time, in combination with adjusting your agitation. If you contrast is too high at 18 minutes, you simply need to reduce time until the contrast isn't too high anymore. 1+100 or 1+50 will not matter much, unless you're getting into developing times of less than 5 minutes at 1+50 dilution, in which case 1+100 makes sense because you then automatically extend the developing time. Sacrifice one roll where you do a series of clip tests, developing at different times. Reduce developing time until contrast is right. That's basically all there's to it, 1+50 or 1+100 matters little; the principle is exactly the same.