According to Galileo, if there were no air friction objects of different masses would all fall at the same acceleration, g if straight down and g cos theta if at an angle.
Therefore I suspect that mass is simply not important here and an error in measuring it, or in fact failure to measure it, has nothing to do with your result (ignoring friction)
Measuring the time can be tricky though. Have you tried it?
What is the biggest source of error in measuring the acceleration of a ball down a ramp?
knowing the length of the ramp
accurately measuring the time
knowing the height of the ramp
knowing the mass of the ball****
3 answers
Yeah I've done a project like this and we had to measure the time with a very precise instrument.
There are ways to do it accurately in physics labs on the subject, but it requires something better than fast eyes and a stopwatch as a rule :) Taking a movie can work if you know the frame rate. However usually you have a paper tape or something being marked every fraction of a second by an electrical spark or something.