There are several things that are useful to know in order to do a convenient 100% run, I'll list them beneath

• You must get the bad ending first before the 'good' ending in order for the 'good' ending to trigger
• Upon getting the 'good' ending, you must score 40k a second time on the original mode to unlock metsu mode
• Once unlocked, aquiring around 5k points on each metsu mode run should unlock every note until the last one for the good ending, I believe this takes 50k to achieve
• Upon unlocking the last note on 50k you should be able to achieve level 99 in metsu thus unlocking the true ending