In Bootstrap, we have default spacers defined, like following,

$spacers: (
    0: 0,
    1: ($spacer * .25),     // 4px
    2: ($spacer * .5),      // 8px
    3: $spacer,             // 16px    
    4: ($spacer * 1.5),     // 24px
    5: ($spacer * 2),       // 32px
    6: ($spacer * 3)        // 48px
);

What if we want a spacer for 20px? Something between spacer 3 and 4. How can we achieve that? Do we have to add it as,
7: ($spacer * 1.25)
OR is there another way?

I tried adding something like below,
3.5: ($spacer * 1.25), // 20px – this throwed error and 3.5 is not allowed.
OR something like this,
3_5: ($spacer * 1.25), // 20px

But would like to know if there any other ways to achieve this.