So the way adaptive works is it only applies to the boost clock. So that means you can set it to use a lower vcore for when it is boosting, but all of the other values in your curve will be the default auto ones.
Manual will set the value across the board without letting it decrease or increase based on usage. You do not want to use this options so it leaves you with one final choice.
Offset mode. If you are wanting to set a value across the board the only other option is offset. So depending on what you are trying to do this might work for you. Offset still relies on taking the voltage requested from cpu (auto) and then it modifies that number. So if for example your cpu is request 1.25V , but you only need 1.22v you can set offset mode to a negative multiplier and have it reduce it by .03v. The only downside to offset mode though is it will apply this setting across your entire voltage range.. so at all steps your motherboard will supply .03v. This can sometimes lead to stability issues and can limit the maximum amount you can cut down on your voltage to reduce temps.
The reason for this is while at max voltage - .03v might be stable, but when the cpu idles and needs .85v and is only getting .82 it can cause issues which is why I said it reduces the overall ability to dial in voltage at max clocks. The last down side to offset is in the case you are overclocking and need MORE voltage.. so you cpu is requesting the maximum it will allow which lets say is 1.3v, but you need 1.32v so you can set a + offset and add on that .02V. The problem is once again that this applies to the whole range. So at idle it is generating more heat due to the fact is is getting more voltage than is needed.
Offset use to be the way to do things in the Sandybridge days, so it isn't terrible, but does have its limitations as do the other 3 options. If you are just looking to reduce voltage that is being requested by auto it can be a good option, but it can also be a little trickier to dial in. I would try to first get what you need at your current max clock with manual. Then see what you are pulling on auto.. type in the - offset to bring you closer to this number and then try a stress test. If it has issues you need to reduce it more.. same for it you have a hang or blue screen when the pc is idle. Just add back voltage until it is stable. This will result in your voltage being lower than auto.
Adaptive is really good for a decent overclock and will limit the maximum voltage used while still maintaining the ability for the cpu to lower its vcore at state changes,
Manual is good for locking in a voltage for constant use. It isn't effiecient, but it is also handy for discovering what is needed for your overclock so you can configure the other two options.
Offset is good if you want to change your entire curve. Handy for a stock clock cpu that you want to undervolt to reduce heat. Also decent for overclocking, but can cause stability issues at different cpu states if you are running really close to your stable threshold.
Remember offset mode is all about finding a balance. I am sure you could play with it enough that you find a happy medium. Remember if you aren't having crashes/stability issues the low idle voltage is fine. If you do have idle issues, just reduce the offset a little and try a gain. Do this until you are stable at the full curve. This will mean your CPU isn't getting hit with full voltage even in idle states.