You cannot test your half-life by comparing two measurements when one of them is so close to the peak. To get an accurate estimate of your half-life you have to compare two measures that are taken at least 24 hours after dosing. Your half-life is most certainly not only 17.8 hours. At 90 minutes your level is close to peak and will drop a lot faster for many hours after which it will then stabilize more and follow the half-life more.
If you want to estimate your half-life it’s better to measure at 48 hours and 96 hours after dosing.
