https://otland.net/threads/exp-scroll-is-giving-50-less-exp.239383/

expscroll.lua

local config = {
	time = 2,
	storage = 200011
}

function onUse(player, item, fromPosition, target, toPosition, isHotkey)
	if player:getStorageValue(config.storage) >= os.time() then
		player:sendTextMessage(MESSAGE_INFO_DESCR, "You still have extra experience time left.")
		return true
	end

	player:setStorageValue(config.storage, os.time() + config.time * 3600)
	player:sendTextMessage(MESSAGE_INFO_DESCR, string.format("You have activated %d hour%s of double experience.", config.time, config.time ~= 1 and "s" or ""))
	item:getPosition():sendMagicEffect(CONST_ME_MAGIC_RED)
	item:remove(1)
	return true
end

player.lua

local config = {
	rate = 2,
	storage = 200011,
}

function Player:onGainExperience(source, exp, rawExp)
	if not source or source:isPlayer() then
		return exp
	end

	-- Soul regeneration
	local vocation = self:getVocation()
	if self:getSoul() < vocation:getMaxSoul() and exp >= self:getLevel() then
		soulCondition:setParameter(CONDITION_PARAM_SOULTICKS, vocation:getSoulGainTicks() * 1000)
		self:addCondition(soulCondition)
	end

	-- Apply experience stage multiplier
	exp = exp * Game.getExperienceStage(self:getLevel())

	-- Stamina modifier
	if configManager.getBoolean(configKeys.STAMINA_SYSTEM) then
		useStamina(self)

		local staminaMinutes = self:getStamina()
		if staminaMinutes > 2400 and self:isPremium() then
			exp = exp * 1.5
		elseif staminaMinutes <= 840 then
			exp = exp * 0.5
		end
	end

	-- Apply Extra Experience
	if self:getStorageValue(config.storage) >= os.time() then
		exp = exp * config.rate
	end

	return exp
end