If you are using catalog system, the way of creating metafields are easy – in fact, you can forget about “metafields”, all you should be using is the typed content type. Adding this attribute to your property is enough to set the precision as you wish.

        [DecimalSettings(10, 2)]
        public virtual Decimal MyAttributesDecimal { get; set; }

Thing is a little different if you are using order system. You don’t have the strongly typed order types to work with. To automate the metafield creation, you will have to use the underlying MetaField API yourself. You probably know how to create a metafield and add it to a desirable metaclass

            var metaField = MetaField.Create(MetaDataContext.Instance, "Mediachase.Commerce.Orders.System.Shipment", "NewMetaField3", "new", "temp",
                MetaDataType.Decimal, 17, true, true, false, false);
            var metaClass = MetaClass.Load(MetaDataContext.Instance, "ShipmentEx");
            metaClass.AddField(metaField);

However, metafield created this way will be added to the metaclass with the default (18, 0) precision, which is kind of pointless. How to control the precision of the decimal metafields?

The little secret is with the MetaField.Attributes. There are two attributes that control the precision of decimal type: MdpPrecision, and MdpScale. You have to set those after the metafield is created, but before it’s added to the metaclass (the reason was simple: the underlying query to add the column to the table looks for those values to set the precision of the column). Your code should look like this

var metaField = MetaField.Create(MetaDataContext.Instance, "Mediachase.Commerce.Orders.System.Shipment", "NewMetaField5", "new", "temp",
    MetaDataType.Decimal, 17, true, true, false, false);
metaField.Attributes["MdpPrecision"] = "38";
metaField.Attributes["MdpScale"] = "9";
var metaClass = MetaClass.Load(MetaDataContext.Instance, "ShipmentEx");
metaClass.AddField(metaField);

A small but important note to remember: both of the attributes must be set, even if you want to leave one to default value.

And Tada!