Hydometer readings

I'm in the primary stage with my first batch of hard cider and plan on picking up a hydrometer in the coming days. It's been sitting for about three weeks and I think I'm ready to rack it and taste it to remove the lees and sample what's to come. What readings am I looking for to make a semi sweet hard cider? I'm going to be killing the live yeast with Camden tablets when I achieve my proper taste. Then bottling and maybe just let it condition and age from there (if I can get enough self control to let it sit.

campden will only knock yeast down, not kill them

You will want to use campden and K-sorbate to inhibit further fermentation