Can do this several ways. I'm still on the old Hub Info but the logic would be the same. Using the trigger as you have it I'd do it with these actions:
So basically triggering on the low memory and then waiting for it to go back over 240 for 10 minutes. Then it either did go over 240 or the 10 minutes expired. So we test and if it's under 240 we do the reboot otherwise it will simply exit. Also not sure but you may need some logic to be sure the rule doesn't keep triggering if the memory keeps shrinking. Like using the Boolean variable and setting it as you are beginning the wait. So something like:
Even if you keep it as you have it now consider the Boolean addition to keep it from potentially running the logic over and over if the memory keeps ticking down.
