How I can make a drop down subject to a result - Updated

[DLM154]

(37) Total System Size = 0 (Column G) kWp

(41) ET: SAP Yield (kWh) x 50% (Column D) x 4.85 p/kwh = £
(42) ES: SAP Yield (kWh) x 50% (Column D) x 16 p/kwh = £


The above is the relevant section I could use some help with.

If G37 is <30kWp, I need D41 to remain unchangeable at 50%, and D42 to be a drop down to change between 25,50,75 & 100%.
If G37 is >/= 30kwP, I need D41 to be changeable as a drop down to either, 25, 50, 75, 100, and for D42 to be the remainder (ie. if D41 = 25% then D42 = 75%)

I currently have it so D41 is a drop down and D42 equals the remainder:
(D42) =IF(D41<25%,1,IF(D41<50%,0.75,IF(D41<75%,0.5,IF(D41<100%,0.25,IF(D41<110%,0,1)))))

I'm unsure how I can make a drop down subject to something.

Please let me know if you can help,

See example below

Many Thanks

[DLM154]

For example if there were something that could acheive:

[D41 and D42 are both percentage number format]

(D41)=IF(G37<30,.5,DROPDOWN(0,.25,.5,.75,1))
(D42)=IF(G37<30,(DROPDOWN(Data)),(IF(D41<25%,1,IF(D41<50%,0.75,IF(D41<75%,0.5,IF(D41<100%,0.25,IF(D41<110%,0,1))))))