Clipping values

I have the following domain defined:

CREATE DOMAIN percentage AS real
CONSTRAINT percentage_check CHECK (((VALUE >= 0.0) AND (VALUE <= 1.0)));
The various values (aa,bb,cc and dd) defined as "percentage" can increase over time, to a maximum value of 1.0. In fact, I have one table with four different "percentage" values which can increase. The plpsql function I'm writing is going to be called from a cron job every 5 minutes to update "aa, bb, cc and dd" percentages every 5 minutes. The amount of the update is a set amount (we'll say .1 for the sake of argument) times the number of five minute intervals (num_intervals * .1) which have elapsed since the last time it was called.

However, "aa + (num_intervals * .1)" might exceed the constraint value of 1.0. Thus, for four different values, I'm looking at ugly IF/ELSE checks to clip those values back to 1.0. This is expected to be a large table and I would really like have this code be simpler. Below is the start of my function. What could I insert into the LOOP to make this simple and correct?

CREATE FUNCTION event_update_percentages() RETURNS void AS $$
DECLARE
last_update TIMESTAMP := (SELECT update_percentages FROM event_manager);

-- 5 minute intervals (60 seconds * 5)
num_intervals INTEGER := (SELECT EXTRACT (EPOCH FROM now() - last_update )::int/300);
item some_table%ROWTYPE;
BEGIN
IF num_intervals > 0 THEN
FOR item IN SELECT * FROM some_table WHERE (
aa < 1 OR bb < 1 OR cc < 1 OR dd < 1
)
LOOP
-- ???
END LOOP;
UPDATE event_manager SET update_percentages = now();
END IF;
RETURN;
END;
$$ LANGUAGE plpgsql;

Better yet, am I approaching this entirely the wrong way? If there is a simpler solution to gradually increase those variables over time, I'd welcome it.

If this is not clear, please let me know and I can try to explain more.

Cheers,
Ovid
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