Motor thrust can vary between batches as specified in NFPA1125 for the same motor designation...... So don't worry about gravity being 9.81m/s^2 Use 10. It's close enough for rough calculations. It's not worth making your rough calculation any more difficult than necessary.
Any scales you will use to weigh things will have been calibrated somewhere on Earth against a known test mass. If you have good scales, they might let you recalibrate your scales where you are. So when you weigh something on scales, it's indicating an equivilant mass......
Any innacuracy in it does not really matter in the real world.
The thrust to weight rule of 5:1 is a general rule. It is set to allow, in general terms, a normal rocket to get to flying speed by the time it stops being guided by the guide rod or rails. The point where it is truly on its own. In the Tripoli rules you could go as low as 3:1 if you could make a case to the Range Safety Officer that it would be safe.
If you go to ThrustCurve.org and look at the data for a specific motor, many will have a peak thrust on ignition. Provided that lasts for long enough to get your rocket to the top of the rod or rail, you can use that peak figure for your 5:1 thrust to weight ratio.
If you look at the curve for the F20W single use
Although it has a peak of 40, 36 or 37 would be a more conservative number to use. It takes a fraction to get there and really doesn't hold it for long.
Where this initial start boost really shows up is in the Estes and others black powder motors. C6 for example has a massive start thrust in comparison to its average. See below. Also note that it is classified as a C6 but has an average thrust of 5 Newtons. A 20% alteration to its designation, depending on how you want to do your maths.
The trust curves are also available in OpenRocket. Once you have highlighted a specific motor in the motor list, you can select the details tab (Top Right) which will show the thrust curve plot at the bottom. If you double click on that it opens up in more detail.
Hope that helps
Norm