How much can holy water be diluted?
The short answer is 49%.
The rubrics for Baptismal Water are the same as those pertaining to Holy Water in this circumstance.
CHAPTER I: RULES FOR ADMINISTERING BAPTISM
- If the baptismal water has so diminished that it is foreseen it will not suffice, unblessed water may be added even repeatedly, but in lesser quantity than the blessed each time this is done. If it becomes contaminated or has leaked out or in any way is deficient, the pastor will see to it that the font is thoroughly cleansed and replenished with fresh water, and proceed to bless it according to the form given below. - Holy Baptism - General Rules
Although these rules are from the 1962 rubrics, they still show that ordinary water may be mixed with holy water and retain its blessing. In other words, it is important to realize that to maintain its sacramental blessing in force, the water added to the original volume is not to exceed 49%. The water added to the holy water must be less than the original volume. Any more than that the holy water looses its sacramental qualities and is to be considered ordinary water.
When adding water a second time to the previous amount of holy water, the water added to that must also be 49% or less of that volume. It should be noted that this is rarely done and that safeguards are generally in place to make sure this does not happen. This should be avoided if at all possible.
This is exact what I was taught in the seminary.
If one desires a large amount of holy water, it would be okay to ask a priest to bless some water in a separate container for personal use at home or elsewhere. I have done this with oil on several occasions.
The 49 %/51% phrase is quite in line with the Church's traditional way of dealing with less than half: 51% Beeswax for example.
It is safe to conclude that the Pascal Candle and the candles used at Mass must have at least two thirds wax in their composition, 67% would be the minimum. About the other candles, the Sacred Congregation of Rites prescribes that a greater (51 %) or notable part of the candles must be [bees]wax. - The Wax Candle in the Liturgy