Gary, girdling will help with fruit retention as well. It should be relatively dry during flowering and then water should slowly be increased as the fruitlets form. As the fruit gets larger, watering should be gradually increased.
If any of your neighbors have Lychee trees, try to observe their flowering and harvesting cycle. As soon as your neighbors harvest their fruit, it should be a good time to trim your trees back. Lychees are naturally large and can get very old. From my observations, if Lychees have too much Nitrogen and water close to flowering, they will drop fruit in preference to vegging out.
Cool weather is a powerful stimulus for flowering from what I've observed here in San Diego as my 1-2 foot tall newly established air layers flower consistently every year.
Simon