Help with formulas to calculate commission post

[Celest]

Hi

I’m trying to work out an appropriate formula to automatically calculate commission. Our commission structure is below;

Commission band £12,500	Percentage
0 – 50%	2.5
50 – 75%	5
75 – 100%	10
100 – 150%	15
150 – 200%	20
200% +	30

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Commission is calculated as follows based on this person sales being £21k for December and there target being £12,500

	Commission band £12,500	Percentage	Commission
£0 – £6,250	0 – 50%	2.5
£6,250 – £9,375	50 – 75%	5
£9,375 – £12,500	75 – 100%	10	1,250.00
£12,500 – £18,750	100 – 150%	15	937.50
£18,750 – £25,000	150 – 200%	20	450.00
£25,000 +	200% +	30
	Total Due December		2,637.50

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If commission falls under 75% the just get the 2.5% or the 5% depending on where it falls but if it’s over 100% they get commission on there target and any amount over the target is -

ie:
75 – 100% is 12500 = 10% commission
100-150% is 6250 = 15% commission
150 – 200% is 2250 = 20 % commission

12500 + 6250 + 2250 = 21000

Thanks in advance for your help