I live in northern California. I have two 'family fun' breed watermelons that I think are ready to be picked. One is small, like a big cucumber and has a yellow patch on the bottom and sounds hollow but does not show any of the other signs. The other one is bowling ball size, sounds really hollow, is very resistant to my fingernail and has a dull light green color. Also, this one has been stored on wood planks. The two seedlings were planted the first of May and we didn't get fruit until late June. It's been about 50 days since then.
Certified GKH Gardening Expert
Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/fruits/watermelon/pick-a-watermelon.htm
If the tendril is green, you should wait to pick the melon. Harvest when the curled tendril closest to the melon on the vine begins to shrivel and dry up.