Unless you're a powerlifter, stay away from seeing how much you can lift for 1 rep.
Unless you're one of those people who take gravity as a personal affront and dedicate every aspect of your training to lifting the heaviest weight possible, you're just begging for a huge ouchie. On the minor side of the injury scale are strained muscles or tweaked spines, and on the major side are broken bones, shattered trachea, ruptured diaphragms, and torn muscles.
I know of which I speak. Several years ago, while fudging around with a bench press 1RM, I tore my pec. And this wasn't some little, "Goodness, I tore a few fibers on my pec and it smarts like the Dickens" crap. This was a complete, phonebook being torn apart by an old-time strongman tear.
The blood pooled up into my arm and down to my elbow, from my pec down to my hip. It changed colors every few days for two months, from the traditional black and indigo to the more nuanced purples and umbers. My naked body could have been used as a display in the paint department at Home Depot. Young couples could point to parts of me and say, "We want the baby's room to be that color."
If I was Marvin the Moose, your dog's favorite chew toy, I could have been re-stuffed and stitched up, made almost as good as new, but it didn't work out that way. The pec tear couldn't be repaired because sewing up muscle is like sewing up wet toilet paper; even the slightest movement causes the stitches to tear loose. So now I have a hunk of gristle and scar tissue where my pec should be.
The Truth About Big Lifts
Look, I know that it's a perfectly natural impulse to see how much weight you can hoist off the ground, but it gets weird when you wrap up your self esteem in how much you can lift. I have the uncanny ability to tell when meat is done cooking just by looking at it, and as mundane a talent that is, it's still more useful to society than your 600-pound squat.
Besides, a big lift is often as much about technique and the sidestepping of physics (going to great lengths to reduce the distance the weight has to travel) as it is strength and power, hence all the comparatively teeny guys on Facebook and Instagram who can deadlift the weight equivalent of a small house.
None of this is meant to disparage a training program like Jim Wendler's 5/3/1 because the calculations are all based on 90% of your 1RM, not your true 1RM.
Do the Math Instead
If your program requires you to figure out how much weight you can lift for one rep, just take your 3RM and divide it by .93. That will give you a pretty damn accurate estimate of your 1RM and it will save you from incurring a really big ouchie.