There isn't a formula that I know of.
That being said on the bike it's not going to help much unless you are going uphill. As mass increases surface area increases at a smaller rate. A 5% increase in mass may yield a .5% increase in surface area. Likewise a 1kg reduction probably won't decrease surface area at all. power/cda is a much, much, much more important metric when racing flat to rolling courses. If it's a hill climb p/wt ratio is much, much more important. If it's a rolling to hilly course then you need to figure out the ratio between flattish to hilly to figure out which ratio (p/cda or p/wt) is going to be more important.

On the run less mass helps to a greater degree when it's hot & humid. The more fat mass you have hanging around the harder it's going to be to shed body heat in hot/humid climates.

A 1 kg wt loss will help for increased running velocity but probably not as much as most on this thread seem to hope for.

On the other hand if you have a kg to lose losing it isn't a bad idea.

If I had to choose between losing a kg and revamping my training a bit to get more run frequency/volume and a 2-3 extra tempo sessions in that 6-3 week period prior to the race without a large increase in fatigue heading into a major race I know what I would choose.

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Brian Stover USAT LII
Accelerate3 Coaching
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