I think the exact calculations will be rounding the ROI% defined as in the script but if you are staking on a pool it will be much higher as you are gathered with other members in the same pool, bringing more weight and contributing more to the network. Thus, you actually might be rewarded more than else staking alone (or more than double if you left the coins staking on the exchange).
You can join a really good and newly created pool (4 weeks old) but fully functional and VERY user friendly https://stakeunited.com/vilgreg (they also work with referrals, so each time a member joins through your link and gains stakes, you'll be receiving a part of the fees that the Pool owners would be receiving (I think 10% of 3% of the fees).

Hope I've helped