| 749R, going from a 15/35 to a 15/38 using the same chain will shorten your wheelbase 12mm. In general, it's better to go to a longer wheelbase than a shorter wheelbase because it improves high speed handling. Superbike corsa swingarms, for example, are 15mm - 25mm longer than street versions for this reason. Shortening the wheelbase beyond the stock setting is really not a good idea.
A good approach gearing is to set the wheelbase at maximum (still allowing the adjuster an additional 10mm for chain stretch) and then use a combination of sprockets and chain length to shorten the gearing.
The length of each link from pin-to-pin for a 5XX chain is 5/8-inch (16mm.) So in your case of using a 15/38, if you use the next longer length (adding two links at a time) you’ll get a wheelbase 4mm longer than stock instead of 12mm shorter. I’d do it. | 
Shazaam!
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