I had built up the patio slipcover project in my head as something that would be annoying and hard, based on my experience sewing lined drapes for the house 10 years ago. Well, I have 10 more years of experience now and a lot better tools, plus I found a tutorial to do the math for me, so this actually ended up being a fun and easy project. 
I wanted a slipcover to be able to wash it throughout the season and I wanted it to be reversible for even wear in the sun. I found this calculator and video tutorial from Sailrite and it was fantastic. I ended up with a perfect fit, just snug enough, and look at this slick zipper installation: 
The tutorial calls for continuous zipper tape but I used a spare separating zip I’d bought for my parka project. It worked well enough but if I make more slipcovers, I’ll get the continuous tape. Because this was so easy, I’m already looking at the rest of the furniture and thinking, “I could slipcover that.”
PS, the pool is a delight as we contemplate a summer of 100+ degree days: