There have been several good threads on this topic. Do a search in the whole forum, because they fall in different sub-forums.
There is much Biblical basis for the 10% or tithe. Depending on your other necessary commitments, you may be able to do more or less. If you have quite a bit of debt beyond house and car payments, you need to be sure to cover those monthly payments because you cannot tithe what you do not have and that money belongs to your creditors.
If your contribution has to vary based on income, that is understandable. It may help you though to figure your average income for the past 12 months and base your contribution on that. Then you can increase or decrease from that amount as your needs direct.
The church does not require a certain amount, but as others have said the need is there for the parish to pay all its bills, be able to offer programs at minimal cost, and to contribute to diocesean charities.