There are some things which you should be aware of while making observations. The first one is that many of the variable stars are quite red. This can make them tricky to estimate. The first consequence of this is that bright moonlight will tend to make them stand out and appear brighter than they really are, so it is best not to observe Mira type variables in bright moonlight. The second consequence is something known as the Pukinje effect.
Red light tends to build up on and excite the retina.
If you stare at a red star it will get brighter the longer you stare. So when estimating red variables it is best to glance at them and then at the comp stars rather than staring directly at the variable to avoid this problem. Weather can play a part in the accuracy of your observations. If there are clouds moving through the field of view, especially high cirrus that you can t always tell is there, this can affect your observations. Your comfort and warmth play a role also.