Postgres closest number?

Question

PostgreSQL

There is a table - the ratio.
That is, for 0.5 - 3
For 1.0 - 5.4
The dependence is empirical (ie calculated in real conditions). The formula does not describe.
Table step - 0.5
How, depending on what the user entered, to pull out the corresponding number?

Smartly, you need to calculate according to the formula

y = ((x - x1) / (x2 - x1)) * (y2 - y1) + y1;

Answer 1

select *  from (
select t.*, abs(value - 1.1) as diff
from   t
order by diff asc  
limit 2
) x
order by value

find the next two numbers

Answer 2

sort by minimum difference and take the first value...

SELECT ...
FROM ...
ORDER @ x-1.3 ASC
LIMIT 1