Scrolling Vim relative to cursor, custom mapping

Question

So I have read :help scroll-cursor and really like zz, which puts the line your cursor is on in the middle of your window.

I'm looking for help to make a mapping that would perform similar to zz but puts the line my cursor is on at 20% of the window height (or 25%, 30% etc).

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Edit:

Thanks to ZyX and Drasill, I was able to modify his function to get the desired functionality:

function ScrollToPercent(percent)
    let movelines=winheight(0)*a:percent/100

    if has("float") && type(movelines)==type(0.0)
        let movelines=float2nr(movelines)
    endif

    let oldso=&so
    execute ":set so=" . movelines
    execute "normal! zt"
    execute ":set so=" . oldso
endfunction

Answer 1

I take it you know about zb and zt which scrolls the current line to the bottom or top, respectively?

Answer 2

This is not specifically an answer to your question, but you can like the scrolloff option.

For exemple : :set scrolloff=5 will always leave 5 visible lines at start and end of you window.

So, when you use zt or zb, your cursor will go 5 lines under top (or 5 lines above bottom, respectively), which can be about your desired 20%.

I personnaly love this setting.

:help scrolloff

Answer 3

Somewhat related, I've got ^J mapped to move the cursor down one line & then recenter the screen:

map <C-J> jzz
map <C-K> kzz

You could subsitute in your scroll-to-percent mapping in place of zz.

Anyway, this has the effect of leaving the cursor in the middle of the screen while text scrolls by behind it - I often use this instead of normal j/k.