When you enter a percentage in this Straight-Line % field, the No. of Depreciation Months, No. of Depreciation Years and Depreciation Ending Date fields will be cleared.
In simple terms, the life of the asset is the start date + Straight Line % in (360 day) years , so you do not need the other fields and that is why they are cleared. 10% = 10 years; 20% = 5 years. There is some change if you have acquistions after the start date.
If you want a fixed end date use the ending date field