I used MFP for a bit, following its idea of suitable carb intake. It was feasible, but weight loss was quite slow. Then I went back to iDave low carb/GI diet and got much better results. But previously, that had lost its efficacy. Something to do with spending time back on the carbs then going back to iDave seemed to help.
But like all of these diary things their main contribution imo is keeping you honest and reminding you what you’ve had. So having the odd biscuit is ok but MFP etc remind you that you’ve already had three or four sneaky biscuits about which you had forgotten.
Is the running calorie estimate accurate?
They never are. To answer all your questions in one – it depends on you, so just trial and error.