See pages 6-7 of the prospectus. The ETN indicative value given one days VIX index return of r is the following:
N1 = N0 * (1 + A + L * r) * (1 - F)
Where N1 is the new indicative value, N0 is the previous indicative value, A is the daily accrual (defined in paper), L is leverage (-1 in XIV...