Setting a random number as you did was a good move since you didn't feel up to doing accurate measurements at the time. Your stride length measurement would alter your distance-walked measurement, but would not affect your number of steps.
Follow the instructions about positioning the pedometer. Mine has a little spot at the bottom that should face down (not up!) to measure accorately. Small details can make all the differences.
note that if you walk very lightly (as when sauntering or shuffling), your steps are not likely to be noticed on the pedometer unless it is very sensitive. So if you're in a store and slowly window-shopping, it's very possible the step-count might appear less than the actual number of steps taken.
There are slight differences in pedometers...and differences in the length of stride we take according to where and why we're walking, so a lot of variables enter into that equation of steps per mile.
Accuracy of the count varies with pedometers, so follow recommendations on your manual or information sheet to get the best results from your pedometer for placement and more. Mine's an Omron that has proven accurate even when carried in my purse, my hand, in my pocket, attached to the waistband of my slacks.
Measuring steps: it's important.
I've gone through several inexpensive ones that never NEVER counted my steps even remotely accurately. But these pedometers didn't have as many programming/tailoring capabilities as the one I now have. Your pedometer wants you to measure your stride, so you have the assurance that careful measurements will help accurate counting.
If your friend is the same height as you and says 'just set it at such and such', ignore the advice. Even people of the same heighth have different stride lengths.
Do measure your steps, following the instructions with your pedometer. I did my measurements four times to be sure of accuracy. Before starting to measure I walked back and forth in the area several times to sort of get a feel for my natural stride. (Be sure to take care to note whether to measure from toe or heal etc.)
If you have a slow, light stride your steps may not be measured simply because the pedometers hasn't detected them.