Linking token bar to warlock spell slots in default 5e Roll20 char sheet

Hello everyone, I've been trying to set up a bar for the token of one of my players' warlock (using the default Roll20 sheet for 5e) in order to make it easy to track spell slots. Here's what I've tried so far. The effect I would like to reproduce is to have a bar where the current value is equal to the value of the lvl1_slots_expended Attribute and the maximum value is equal to the current value of lvl1_slots_total (hopefully editing spell level by hand as the character levels up). I tried to add @{lvl1_slots_total} as an optional maximum value to the lvl1_slots_expended attribute, but it doesn't work. In fact, if e.g. said character has 1 spell slot left out of 2 ( lvl1_slots_expended is set to 1 and lvl1_slots_total is set to 2), the resulting bar looks like it has 1 maximum, i.e. it looks full even with 1 spell slot left, and the tooltip reads " 1/@{lvl1_slots_total} ", so it's clearly not referencing the Attribute correctly. What am I doing wrong? Thanks in advance for any help anyone may provide.

There are limitations to the token bars. To represent a current and a maximum is not a simple matter of attributes, it involves behind the scenes character sheet coding.  My suggestion is you input the maximum manually. On the plus side, Warlock maximum spell slots rarely change. 

Since Warlocks only have one level worth of spell slots, I use one of the "Class Resource" boxes on my character sheet for it.  I also set it up to reset after a Short Rest.