Note: Not a solution but marked as such to elevate a key reply
Hi @Thijs_Sturm , welcome to the forum 
With the Dashboard tab that was recently added to Portfolios you could create a chart based on your projects in the portfolios and select the count by your numeric custom field. This works best using ‘donut’ or ‘number’ type charts so the total will be visible.
Not as ideal as having a sum at the bottom of your Portfolio’s List view, but definitely more visually pleasing 