If it takes you a few days to use 40% of your battery and you do not foresee the need of extra range unexpectedly, then reduce your daily limit to 60% to further reduce the stress on the battery. However, in my uneducated opinion, the difference in stress of charging from 60% to 70% is very small. Going from 70% to 80% the stress jumps up much greater. Going from 80% to 90% the additional stress is even GREATER, etc. If 70% works for you, then just leave it there until the day you actually do need the miles that will take you below 20%. In these cases, charge to 80%, or even 90% or 100% for that day.
Additionally, discharging below 30% will cause stress on the battery. The lower the discharge the greater the stress. This is why I plan for 20% reserve when on trips. Those VERY few times when I have gone below 10%, once even down near 0%, it did cause stress on the battery, but not enough to worry about. Now if I did this EVERY DAY it would begin to cause a lot of stress on the battery.
This is why the 50% state of charge, and the daily range be +- around the 50% mark, is the ideal place to be for long term health of your battery. For me, I plan to keep this Model S, and our Model 3 D that we are waiting on, for the 1 Million miles promised by Elon.
We had our 2013 Nissan Leaf for over 3.5 years and 50,000 miles. In that time we left the charge setting at 80% daily and only charged to 100% about once per month for battery cell balancing. During that time our range and battery capacity only went down less than 5 miles. We were very happy with the Leaf as a daily commuter/local driving vehicle and had planned to keep it "forever" (meaning until we got our Model 3 and passed the Leaf on to our daughter). But, alas, that Ford F-150 that smashed the left rear of our baby ended those dreams. jmho