<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">The ribs were overdone however. Some bones actually were falling out!Is there a rule of thumb I should be following for cook times? </div></BLOCKQUOTE>
Not really. Times can vary depending on actual cooktemps. If you establish a method of cooking you like, and temps you prefer, then repeating that approach each time, and using Mark's suggestion to learn tenderness by feel, you'll soon learn to estimate cook time fairly accurately based on your previous experiences along with any variations you note during the current cook. (Consistent-approach cooks don't vary all that much but they can and do vary.)
Me, I cook both spares and backs at substantially higher temps than most (325-350+) and ignore set times - I don't think in terms of time at all. I cook till the ribs are richly colored then, if foiling (which I often do, but not always, to add a flavor layer), I foil meat side down with a splash of reduced juice(s), the cook till tender while in the foil. Back on the grate for several minutes, unfoiled, to firm the bark: done. No need to watch a clock. Since the ribs are always cooked till tender the results - tender ribs - are the same from cook to cook, even if the time varies a bit.
Though I prefer higher temps the same or a similar approach can be used with any cooktemps you choose.