create temporary table mean as
select
  test_id,
  avg(point) as meanpt,
  count(1) as count
from test_results
where test_id = '100'
group by test_id;

create temporary table stderr as
select
  '100' as test_id,
  sqrt(sum((r.point - mean.meanpt) * (r.point - mean.meanpt) / mean.count)) as err
from test_results r
inner join mean
on r.test_id = mean.test_id;

select
  r.user_id as USER,
  r.point as PT,
  round(50 + ((r.point - m.meanpt) * 10) / e.err, 1) as DEV_VAL
from test_results r
inner join mean m
on r.test_id = m.test_id
inner join stderr e
on r.test_id = e.test_id
where
  r.test_id='100'
order by DEV_VAL desc, r.user_id
;