Dependant default value of leaf

I have two leafs a and b. I want default of b as 0.8 times configured value of a. Is there any way we can do that.?
leaf a {
description “xx”;
type uint8 { range “0…60”; }
default 5;
leaf b {
description “xx”;
type uint8;
default 0.8 * value of a;

There is no way to do that in the data model. What you should do is to describe this relation in the description statement for leaf b, and then implement it in your instrumentation code. E.g. in a CDB subscriber that is supposed to actually do something with the value for leaf b, if there is no value set, compute the value from leaf a and use that.

There exists a related extension tailf:default-ref (see the tailf_yang_extensions(5) man page), but it can not express a computation, only a direct reference. In theory, you could add a leaf, say b-default, use tailf:hidden full for that and tailf:default-ref ../b-default for leaf b, and implement a set-hook or transaction-hook for leaf a, that sets b-default to the desired value. But this is way more complex than the method above, and doesn’t add any significant value.

Thank you @per for your response.
Tried your suggestion of setting value of leaf b at back-end implementation.