The distance you position your fixed crawl nock below your usual nock will depend on your facial structure and your anchor, so will vary, so you will have to experiment. After you determine where it should be, you can replace the separate fixed crawl nock and your lower regular nock with a continuous serving, if you wish. Mine is about 19 turns of .03 Halo, or about 7/8”, for a 20 yard crawl. I dedicate that bow for fixed crawl hunting only, so I retuned the arrow at the fixed crawl position, which meant raising the upper nock 1/8” or so. If you want to do that, you should do it before you go to the trouble of serving the whole fixed crawl distance, as there will some back and forth changes until you get everything the way you want it.
You will be using the tip of the field point or broadhead to aim, so the closer you can get them to the same length, the less change there will be in POI between the two.