It can be done, because I've done it, but it's not something I recommend, either. You're treading a very thin line, because if you let FC slip and get algae, you'll be forced to drain or buy bleach by the barrel. Minimum FC should be 8 or above, so a regular test kit is useless. You have to use the FAS-DPD test to be accurate, so I hope you have plenty of refills waiting.
I understand the reluctance to waste water. So do what I do: use pool water on the lawn, then refill the pool. I move valves around and empty the spa into the pool, then refill the spa from the hose while using pool water on the lawn. And then set everything back to normal and it mixes fine. In my case, a spa-load of water plus another inch or two from the pool will water all the lawns quite well. And replace about 5% of the water each time.