Help - Tracking sheet with sum/difference formula - reset value

So I have created a spread sheet to track hours based on how much time has passed. To clarify, the only values I will ever enter in the spread sheet is in cells B4-B45 and C4-C45. So essentially when I enter a value in those cells, across that row it will automatically subtract from the baseline I have in cells D3-L3. Here is an example; the formula in cell D4 is =IF(ISBLANK(B4),"",D3-B4). The whole spreadsheet works fine, but each column I'm tracking is based off of a scheduled increment. So every time I enter a value into column B, it will subtract from column D, which the increment I'm using is 30. So eventually when I go down in rows and column D is down close to 0, I want to be able to reset the value. An idea I was playing with was creating a drop down in column M. My question is: is there any way I can create a drop down list column M, and based off of which value I select in the drop down it will copy that value to another cell. Specifically, if I had a drop down in M4 consisting of 30, 25, 50, 100, and 120, is there any way I can make it to where if I select 30 it will copy 30 to D4. If I select 25 it will copy to F4 etc. Let me know if there are any ideas. Thank you!

Hi
please post a sheet. Pics cannot be worked with. Thanks

Of course, attached is the document.

Update to my dilemma, after messing around with it for a while I came up with a formula that suited my needs just in case anyone reading my thread would find it useful. The only thing is I had to create a reset column for each column I was tracking using the CONCATENATE in my formula. Here is a sample formula I updated in D4: =IF(M4=30,CONCATENATE(M4),(IF(ISBLANK(B4),"",D3-B4))). So essentially anything I enter into B4 will be subtracted from D3, but if I want to reset the cycle I just enter my baseline (which in this case is 30) into cell M4 and it resets it. If anyone has an idea that would work better though I would love to hear it. Thanks again!

What is the CONCATENATE for? This function is used to join to or more text strings. If it contains only one reference, it does not do what it's meant to?

As for the rest, I don't understand what you are trying to do

The overall goal I was going for is for the value in the B & C columns to subtract from columns D-J. I used the CONCATENATE to use a reset column in column M. So I can keep the tracking sheet going, and once I need to reset, for example, column D back to my baseline which is 30, i just use column M to enter the value 30 and column D will mirror it without erasing the formula. I wanted to use something like this instead of manually putting that value into column D because I plan on having everything aside from columns B, C, and M locked, so it can be repetitively used. But like I said if you have any better ways to do this I'd love to learn! Most of this I just recently learned from research.

Something like this ? =IF(ISBLANK(B4),"",MAX((D3-B4),M4))