It’s hard to judge without knowing what brand of masking fluid you use. On average I would use a ratio of 20% water and 80% masking fluid. If you add too much water to the masking fluid, it will soak into the paper and will tear the paper when you try to remove it. When you dilute the masking fluid, add it to a second bottle, and then only add the water. Also do not pour water into the original masking fluid as it will become runny and unusable.