The current method of calculating Kt/V is to consider daily drain volume * D/P urea (which gives the peritoneal Kt) and then divide by V (which is estimated by using one of the formulas for body water such as the Watson formula).  This will give the daily peritoneal Kt/V which is then usually multiplied by 7 to convert to a weekly Kt/V.
If one wishes to calculate Kt/V for a less than 7-day prescription, the daily Kt/V can be multiplied by the number of days for which dialysis is done.  Alternatively, if one is provided with the weekly Kt/V (which assumes a 7-day prescription), one can then multiply that by the percent of days of the week the individual is doing PD (i.e. multiply by 6/7 for a 6-day prescription, 5/7 for a 5-day prescription, etc…).
This will give the weekly peritoneal Kt/V which then would then be added the total weekly renal Kt/V for the total weekly Kt/V