How to compute the response.headers size?

[Jhc-china]

I don't know how to compute total size approximately
though the header size is always between 286 and 288 B, in my project
i want to compute it, not use a const value(287 B) instate
Anyone can help my to solve my problem?
( p s: it's pretty easy to find response.content-length, ctx.length can do this job,
but the total length is body.content-length plus headers.length)