You can create an asset distribution in 2 ways:
When you create an asset distribution, by default it will inherit the "Value Now" of the asset (see Determining Asset Value). Any changes you subsequently make to the asset's "Value Now" will automatically be reflected here, and any changes you make to the distribution's "Value" field will update the asset's "Value Now". Once you mark a distribution "Done", no further changes are allowed here (unless you are dealing with multiple partial distributions, one or more of which is not "Done": see below).
While the normal case is to distribute a given asset to a single heir, sometimes it is necessary to split the asset among multiple heirs (or distribute portions of the asset to the same heir, but at different times).
To enter a partial distribution, click the "Partial" checkbox above the Value field in the top right of the Distribution dialog, and enter the percent of the asset you wish to associate with this distribution in the "Allocate %" field.
Sometimes it can be difficult to get the percentages to add up to 100% (for example, if the will divides things equally among 3 heirs). In such cases, it is usually best to slightly modify the percentages to get to 100% exactly, or else EstateExec will think there is still some portion of the asset left to distribute.
In our example of 3 equal heirs, just set the percentages to 33.333%, 33.333%, and 33.334%. Once you mark a distribution Done, you will be able to override the actual distributed value in the bottom right of the Distribution dialog: change this value to be exactly the amount you want so that everything ends up equal, or if necessary, one or two of the heirs get an extra penny or so.
Some minor notes about partial distributions:
An asset's cost basis is proportionately allocated to each distribution, so that the heirs can understand their cost basis when and if they eventually decide to sell the inherited asset. For example, if the heir inherits 20% of an asset, 20% of the asset's current cost basis will be recorded with the distribution. See also Manage Asset Cost Basis.
Once you mark a distribution "Done", the item is considered to have been physically delivered to the heir; until then, it is only allocated (e.g., assigned) to that heir.
If it's an asset distribution, the asset's "Value Now" in the Asset table becomes blank since the asset is no longer in the estate. If it's a partial distribution of an asset, only the percentage of the distribution marked "Done" is removed, and so there may be some remaining asset "Value Now" until the rest of the asset is distributed.
A subtle but important point is that if the asset's value and/or cost basis fluctuates during the time between one partial distribution and another, the same percentage of the asset may end up being recorded as different distribution values. This is correct, but can cause complications in the real world, so it's best to distribute all portions of an asset simultaneously. Once you mark a distribution "Done", changes to its asset's remaining Value Now or Cost Basis will only affect remaining partial distributions that are not yet "Done".