I want the power level of the setting to be one of level 1-20 and not more so the maximum CR would be CR 23.
You place FAR too much faith in both the CR and EL systems. CR is not a mathematical formula - it's a best guess. EL, being based on CR, is no better and in fact
breaks down at the lowest and highest ends of the chart and becomes rather unreliable.
The CR ratings serve two purposes - to
try to make sure that PC's can be challenged with reasonable expectations of predictable outcomes (no unexpected cakewalks or TPK's) and to calculate the experience awarded. But determining the CR itself is not based on a calculation - it's based mostly on comparison with other monsters of a similar caliber. There isn't a chart or a formula that says, "if it can do this it's worth this much in CR, if it can do that it's worth that much."
Facing low CR monsters at 1st level PC's are at the most vulnerable they will ever be in their careers. The luck of the dice has a magnified effect upon encounter outcomes. Player skill and the DM's ruthlessness in encounter setup also factor in to a higher degree than they otherwise would. Simply having lived through several encounters and having money to buy slightly better equipment will make a difference.
Once the PC's are mid level, IME you can actually throw monsters at them with a higher CR than the book says they'll be able to survive - SOMETIMES. Depends on the monster, circumstances of the encounter, etc. Here their equipment is of greater significance, but still the very nature of how the encounter is set up can be a huge factor, regardless of the actual monster stats. Any numerical advantage that the PC's have makes a huge difference because if a monster is fighting alone against the party it almost always has a limited ability to inflict damage on EVERYONE. The KIND of characters that players have built will make a difference. It may indeed kill one or two PC's but monsters tend to succumb easily to being ganged up on. Players also have a deeper well to go to - they can pull out ALL the stops and unleash powers and equipment that they just haven't had need to use when the vast majority of what they've BEEN facing hasn't required a maximum effort. Having time to prepare to face a high-CR foe and buff everyone to the hilt is different than stumbling into an encounter and having to scramble.
The EL system is just a TOOL for DM's to use in crafting encounters it is not the be-all/end-all of encounter creation. You ultimately have to ignore it in favor of what YOU, the DM, know of the capabilities of your players and their characters. Even then they'll surprise you.
I say all this only to support the idea that there is no hard line of what CR is acceptible and what isn't - and no reason to hamstring yourself when the campaign gets to those uppermost levels. Indeed, at that point the EL system is becoming rather unreliable in determining the makeup of a lot of your encounters. Average encounters will still be average encounters - but the upper limit of what they can handle is likely to be rather higher than what the SYSTEM says they can handle.
