Saving Weight

[Keep it stiff]

Does anyone have a formula or an estimate methodology that converts weight saving into % of improved track time?

I'm thinking about having custom aluminium bonnet and boot made to replace steel originals and I could go further with other parts. Not cheap of course however I think I could get a 50 kg - 100 kg off a 900 kg car depending on how far I go, how worthwhile could it be?