The solution, Mercator projection:

local function mercatorY(lat)
    local radLat = math.rad(lat)
    return math.deg(math.log (math.tan(radLat) + 1/math.cos(radLat)))
end

And it will be called as

        local x = tonumber(nodeStr:match('lon="(.-)"')) -- X
        local y = mercatorY(tonumber(nodeStr:match('lat="(.-)"'))) -- new code

and

    local minY = mercatorY(tonumber(minLat))
    local maxY = mercatorY(tonumber(maxLat))