Just suppose that you have to achieve “50 transactions per second in avg. response 0.5 sec for
100 users in 1 hour” as goal of load test,
so how much pacing or think time we should give in our script.

1 hour= 3600 sec

Expected Total Transactions = 50*3600=180000 trns/hour by 100 Users

Each user will perform 180000/100=1800 transactions per hour

1 transaction is taking avg 0.5 sec so 1800 transaction will take = (1800*0.5)/60=15min

Now think time will for this run will be =60-15= 45 min =2700 sec

1800 transactions in 2700 sec so one transaction will take = 1.5 sec (this think time we need to include)

Total time to complete one iteration = 0.5+1.5=2 sec (avg)

Lets cross check our calculation:

1800 transaction * 2 sec= 1 hour (Achieved)

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Pacing Calculation :

D = Duration of the test(test window/time frame)

B = Baseline time(total time taken by 1 Vu to complete 1 whole iteration)

T = Total amount of Think time in the script

I = Expected/Target iteration

R = Residual time of the test window.

R = ( D - (T + B)*I)

P = Pacing interval

Dividing the residual time by target iteration gives pacing interval

Hence:

P = R/I

D is pacing time.
(T + B)*I represents the duration of a scenario
and P is the waiting time before the next scenario

Pacing Calculation for your reference

SLA defined/Data supplied by business team :

a.    Number of users = n
b.    Transactions/hour = t

Calculate Transactions/hour/number of users(k)=t/n
Pacing required (p)= 3600/k
 
Here pacing should be variable between  (p+0.1p) to (p-0.1)p

 
For example :
a.    Number of users=20
b.   Transactions/hour=80
Transactions/hour/number of users k=80/20=4
Pacing P=3600/4=900