mysql – adding alias columns as a new column

I have the following mysql query…

select x.timest,     
    max(case when x.devloc='outside' then x.value end) as outside,     
    max(case when x.devloc='hvac_main_return' then x.value end) as hvac_main_return,     
    max(case when x.devloc='hvac_main_supply' then x.value end) as hvac_main_supply     
from sample x where date(timest) = curdate() 
group by timest     
order by timest desc;

It gives me output like this:

+---------------------+---------+------------------+------------------+
| timest              | outside | hvac_main_return | hvac_main_supply |
+---------------------+---------+------------------+------------------+
| 2021-01-28 23:59:54 | 24.8000 |          67.4375 |          82.9625 |
| 2021-01-28 23:58:45 | 24.9125 |          67.1000 |          80.8250 |
| 2021-01-28 23:57:42 | 24.9125 |          66.0875 |          78.2375 |
| 2021-01-28 23:56:33 | 24.9125 |          64.9625 |          74.8625 |
| 2021-01-28 23:55:32 | 25.0250 |          62.3750 |          73.0625 |
| 2021-01-28 23:54:17 | 25.0250 |          62.8250 |          74.7500 |
+---------------------+---------+------------------+------------------+

Does anyone know how I can add the difference between hvac_main_return and hvac_main_supply as another column?