It depends on the ritual. It would differ significantly depending on what is trying to be accomplished. For a fertility ritual menstrual blood would likely work well. If the ritual involves a spirit or deity, I would check to make sure the substitution would not cause offense. Generally substitutions should be left to your best judgement. Remember, experimentation leads to discovery!
You likely don't need a lot of blood for any ritual. I find most deities are understanding of our limitations and struggles to do rituals. I've never not had results from lack of blood, or not being able to do it step by step. I use a blood sugar thing to prick my finger and get blood, you can do it to multiple fingers. Doesn't show up to others, but it can be sore if you squeeze to get more from the finger.
Using menstrual blood can cause your spell to end up different because of the nature of the blood. Although you could plainly state that it is substitution for normal blood.
I'm just gonna give advice but your parents won't think you're depressed if u have small little holes in your fingers
Also that's disgusting do not use menstrual blood for a ritual
Try using a sewing needle and poking your fingertips for a drop of blood
If you feel uncomfortable with that dont use blood or just find another spot
Be careful and good luck