Hello, you can try to use lua codition https://wow.gamepedia.com/API_UnitCastingInfo:
local name, text, texture, startTimeMS, endTimeMS, isTradeSkill, castID, notInterruptible, spellId = UnitCastingInfo("player")
if name then
local finish = GetTime() - startTimeMS/1000
local casttime = (endTimeMS - startTimeMS) /1000
local p = finish * 100 / casttime
print(name .. ' casting ' .. p .. ' %.')
end