I strongly suspect that you should wait till he gets back, because mattresses come in such varied sizes it's going to be impossible to get this correct. I suggest that you make it as big as you want, don't bind it yet but have at least 12 inches of binding ready to put on when and if needed.
When we went hunting for a new mattress in about the 1980s, the whole thing confused us both. There were even then so many king sized ones we almost told them to just pick us out one and send it to our house.
There were baby mattresses, cot sized mattresses, bunk and twin sizes..and each of those came in regular, long and extra long. Then there was the double bed, and a standard bed..we never could understand the difference, if any between those two. And then there was the Queen bed, and one with an extra long mattress.
There was the usual king. A California King, an Eastern King, an extra long King, and at least one other I don't remember. And then there were the different brands, each one of which had a different depth, which now could range from that 8 inches deep to 20 for all I know. And that pillowtop? Who knows?
And one of my sisters and her DH had a mattress built to fit their much loved antique bed that had been hand made by a master carpenter. That mattress was different and she never could find sheets that fit well.
Better wait till he gets home to finish it, is my suggestion.
And besides, it's possible that they might later decide to get a King sized bed to match a new set of bedroom furniture.