Use a 10 to 20 Force (see Mark Wilson's Complete Course). Ask them if the number is a single digit (it will be, but asking indicates that it is possible to have any number). If the answer yes (if it's no you've got a problem), then have then double the result. Depending how you are doing the SCAAAFN (selected card at a forced number), you can have them multiply their number by 2, 3, or 4 because using any other number might exceed 52 (OK it won't, but that will fly over their head) and have dupes of the selected card at 18, 27 and 36. That gives it more random feel.
Dr. Bill has a book entitled THE FORCE that covers the psychological forcing of two specific sums and how to use it, I use them frequently for one of my Seance set ups (a wedding date) But there are other ways of insuring the 37 choice if you aren't comfortable with the standard pitch; offer two different ranged forces to two different people. Ask the first to chose between 1 and 5 and another between 5 and 9 . . . don't get much easier (haha). . . seriously, you can create a ranged force of this sort. Take a look at Rick Maue's TERASABOS and THEN THERE WERE THREE. and you'll find a proper solution that's quite empowering.