It not tested but I think it works:
SELECT T2.SlpName, Count(*) as 'Quotations Converted to Orders', (select COUNT(*) from OQUT t1 where (T1.SlpCode = T0.SlpCode and T1.DocDate BETWEEN [%0] AND [%1])) as 'Total Quotations' , Count(*) *100 / (select COUNT(*) from OQUT t1 where (T1.SlpCode = T0.SlpCode and T1.DocDate BETWEEN [%0] AND [%1])) as 'percentage'
FROM OQUT T0 INNER JOIN [dbo].[RDR1] T1 ON T0.DocEntry = T1.BaseEntry inner join OSLP T2 on T2.SlpCode = T0.SlpCode
where (T0.DocDate BETWEEN [%0] AND [%1])
Group by T0.SlpCode, T2.SlpName