Make it your habit to give generously, even extravagantly, as the Spirit liberates you.
If you are sitting there calculating, then you probably haven't found a liberty in the Spirit that is surely possible to discover. People seem to think that it is a percentage of their income, yet many of us in the West enjoy an excess so why not give generously from that excess and live off just what you need? For some with ample means that may translate to keeping 10% and giving 90! Again it is not the percentages that are the point, but your Spirit-enabled ability to image God's generosity in your own life.
Proverbs chapter 3 verse 28 tells you to give to your neighbour straight away rather than delay, so a further principle is to give promptly as soon as you have the money at your disposal. So if, for example, you are made redundant and receive a lump sum - then given generously from that as soon as you have received it and allow yourself to trust God for your own ongoing means.
Adopting this simplicity witnesses to the freedom that comes in becoming a follower of Jesus. You are not taking up adherence to rules, percentages or the like, but discovering the enabling liberty that God always intended. Your attitudes to these things, and the habits you adopt to work them out in practice, will be directly linked to this freedom.
It surprised me afresh that when Noah dis-embarked from the boat the first thing he did was worship through animal sacrifice. He did not calculate or hoard - for if he had then surely he would have kept the animals 'just in case'. No - he worshipped and freely gave them up, trusting in God for the outcome in the long term.
Find the same ability to 'give up' in worship, and witness to freedom!