Totally agree with the response form Steve. I have been on projects where the minimum requirement has been tonnage and 100 mt seems to be a favourite and also on projects where it is a unit of weight increase. On one of those projects the operator's procedures required a 0.5 ppg increase and the drilling contractor's procedures had a full 1.0 ppg rise. Ensure that the calculation is for a full circulating system (active pit, hole volume, flowline and sandtraps) and use the maximum fluid density expected for the section in question as the starting weight for the calculation if using a weight unit increase. Make sure that you take into account any potential dead volume in the Barite silos and remember that this calculation is your minimum contingency volume not the overall requirements for weighting material for the section in question.
As already highlighted drilling fluid requirements in exploration wells have greater unknowns over development wells where a reasonably accurate dilution factor can be determined. As with all wells much depends on the type of formation that is being drilled, the size of the drill cuttings (a volume of smaller cuttings has an overall larger surface area than the same volume of larger cuttings therefore more mud/volume of cuttings is lost), washouts, faults/fractures.