Hi, Eva. So sorry you're going through this with Hannah.
We had an English setter with chronic heart failure with a heart murmur and cough. One of the drugs prescribed for her was lasix. As I recall, they started her out a little lower than they expected she'd need, then up'd it a bit. Unfortunately, I don't remember the dose. The last prescription we had for her listed the strength at 50 mg per tablet--but I can't recall if we broke it in half or if she got 50 mg/day. She weighed about 45 lbs. So 40 mg/day in a 16 lb dog does seem a bit high. It could be, though, that your dog has more fluid build-up than Ridge had and needs a higher dose.
If you have concerns about the dosage, it's probably worth a call to the vet clinic to see what the vet's rationale was prescribing it at that level. I'm surprised that they didn't do at least a baseline renal test so they'd have something to compare to. We took Ridge in every 3 months for a renal function screen. She was also on Vetmedin and Linisopril, though, and one of those might have been the reason for the frequent screenings.
Btw, she was on the meds for 3 years and died of causes unrelated to her heart problem, so the meds can be quite effective. I hope Hannah improves and is with you for many more years!