I looked at the photos of the Sail 150 at PSM. The lower shock mount seems to be a single grommeted mounting ring, so it doesn't have the long fork to drill new holes for a shorter mounting position.
But the good news is that it is a simple fixed mounting position, so a shorter shock should work.
Check out this 10.25" shock from Amazon ($30). If the mounting posts have compatible sized holes, it should work perfectly. You can dial up to 4 stiffer spring preload settings as well. 30mm shorter
Attachment 28306
google this description to find it on Amazon
RUTU 10.25" 260mm Rear Shock, Mono Shock, Universal Pit Bike Shock Replacement for SDG SSR Pitster Lifan 110cc 125cc 140cc 200cc 250cc Pit Dirt Bike 400LBS - Black
How much will this lower the bike? Well, ask a mathematician. I am sure it is a Trigonometry problem since the swingarm makes a circle when it pivots.
*If you disconnect the shock and do manual measurements with the bike on a bike stand, you can estimate the effect of a 30mm change.