When do I weigh my baby?

She was 2.5 months when we got her in August and she ate for us, but it took 2 weeks to get a huge liquid pee, a urate and a small poop.
Then, she continued to eat and it took another 2 weeks to get a big liquid pee, a urate and a tiiiiiiny speck of poop.
Now, she's 3.5 months, still eating and after 2 more weeks, no poop, no liquid pee, just one small urate.

Do we weigh her after we find a urate? she is obviously utilizing all of her food and most of her liquids, but do I get a correct weight after a urate then?

My adult does huge pees (liquid and urate) about every other day and poops 1-2 times a week, so we KNOW when she is empty and can be weighed.