I want to cook a recipe that says to cook at medium-high for eight hours, however my slow cooker only has high and low. How do I cook my recipe so the meat doesn't resemble old shoe leather?
If it's a wet recipe (casserole, pot roast) you're more likely to find the meat falling apart than tough.
Medium is non standard and there's actually quite a bit of variation between slow cookers. Medium on our current model is pretty much the same as high on our old one; high on the new one is a bit fierce for most recipes without adaptation.
I suggest that you cook it on high, but if you're around when it reaches the minimum cooking time, then turn it down to low (not warm) until you're ready for it.
The good news is that the low and high settings are probably not far apart. In "Slow Cooker Revolution" (2011, The Editors at America's Test Kitchen), the editors remark that you can test your slow cooker by cooking 4 quarts of water for 6 hours on either the high or low setting. An instant-read thermometer should register between 195 and 205 degrees, regardless of the setting you chose.
What does that mean for you? Using the "high" setting on your slow cooker will not adversely affect the final product, but do expect the cooking time to be quicker than the recipe states. Based on my own experience, I'd suggest that you start taking the temperature of the meat with an instant-read thermometer when you've reached 2/3 of the stated cooking time. If the recipe gives a range of times (4-6 hours rather than just 6 hours), then just start testing at the beginning of the given range.